Advances in cellular and molecular immunology have assisted in defining the mechanisms operative in immunemediated tumor elimination and developing immune-based therapies for human malignancies. The eradication of leukemia after allogeneic hematopoietic stem cell transplantation (HCT) is in large part mediated immunologically by effector cells contained in or derived from the stem cell inoculum and represents a remarkable demonstration of the curative potential of immunotherapy in humans. There is substantial evidence that the immune mediated graft-versus-leukemia (GVL) effect results from elimination of residual leukemic cells by donor T cells specific for recipient minor histocompatibility antigens (mHAgs). The GVL effect can occur in the absence of graft-versus-host disease (GVHD) suggesting that a subset of mHAgs expressed on leukemic cells and recognized by donor T cells are not broadly expressed in the nonhematopoietic tissues that are targets of GVHD. However, our understanding of the molecular nature of mHAgs in humans and their distribution of expression in tissues is rudimentary and has impeded the development of effective strategies for eliciting a potent GVL effect without GVHD. Studies supported by this project have developed methods for isolating CD8+ and CD4+ mHAg-specific T cell clones after allogeneic HCT and used cellular and molecular approaches to identify the genes encoding mHAgs and define their expression on leukemic stem cells. These results have led to a clinical trial that has demonstrated the feasibility of isolating and adoptively transferring mHAg-specific T cell clones to augment GVL responses after HCT. In this proposal, we will build on our efforts to understand the role of mHAgs in the GVL response and to develop specific T cell therapy to treat persistent or recurrent leukemia after allogeneic HCT. The specific aims are: 1. To identify genes that encode minor histocompatibility antigens (mHAgs) recognized by CD8+ and CD4+ T cells after allogeneic HCT. 2. To determine in patients with posttransplant relapse of AML and ALL, the safety and antileukemic activity of therapy with T cell clones specific for mHAgs that are selectively or preferentially expressed on hematopoietic cells. 3. To evaluate the H-Y antigen UTY as a target for GVL responses after sex mismatched HCT.